This invention concerns a method for the controlled pre-rolling of thin slabs leaving a continuous casting plant, and the relative device.
To be more exact, this invention concerns a method and relative device for controlled pre-rolling, carried out on thin slabs leaving a mold for the continuous casting of thin slabs, immediately downstream of the foot rolls of that mold and concerns also the device suitable to apply that method.
By thin slabs are meant slabs 800 to 2500 mm. wide, or more, and 25 to 90 mm. thick.
The invention is applied advantageously, but not only, to slabs having a final thickness between 30 and 60 mm. at the outlet of the continuous casting machine.
The invention can also be applied to the continuous casting of billets, whether the billets be round, square, rectangular, etc.
The invention can be applied to straight and curved continuous casting plants.
Pre-rolling methods are known whereby a thin slab undergoes a pre-rolling action in a zone distant from the foot rolls.
In the methods of the state of the art disclosed in JP-A-130759, U.S. Pat. Nos. 3,891,025, 4,056,140 and 4,134,440 the pre-rolling does not give satisfactory results inasmuch as it is performed in a position of the slab such that the liquid core or pool is only seldom still present and the skin of the slab is anyway already thick and is such that it cannot be readily deformed.
Moreover, the skin of one side of the slab is connected to the skin of the other side by columnar solidification elements at an intermediate position between the edges; both the edges, which contain a consistent solidified thickness, and also the columnar solidification elements offer a strong resistance to any alteration of the thickness of the slab.
The pre-rolling in the methods of the state of the art, therefore, has only a marginal and very limited effect, which does not give the envisaged results.
Moreover, the pre-rolling as carried out in the methods of the state of the art has only the purpose of performing a marginal superficial work, and the real work of reducing the thickness of the slab is entrusted to the rolling train located downstream.
Furthermore, in the pre-rolling assemblies of the state of the art only some pairs of rolls are controlled to check the pre-rolling parameters, and these pairs of rolls are controlled in a differentiated or separate manner (see JP-A-130759). This has the result that the operating method cannot be conditioned according to specific requirements but is quite random in some ways.
Pre-rolling assemblies are also known which comprise mechanical adjustment systems together with pre-adjustment at the beginning of a casting campaign. These assemblies are associated with pre-rolling rolls in continuous rows or divided into sectors or groups of rolls or else with assemblies of pressure belts.
These systems of the state of the art do not make possible excellent adjustments, nor a substantial pre-rolling action of a desired value, nor a continuously controlled travel continuously related to the actual pre-rolling requirements. They also do not enable processing to be carried out on a zone where the liquid core or pool is still substantial and the surface skin is still of very small values.
The state of the art prevents controlled mechanical action to reduce the length of the liquid cone and thus to ensure better quality.
Moreover, the state of the art entails considerable limitations with regard to problems linked to the transient conditions of starting and stopping and does not permit an excellent yield of the system.
The state of the art does not obviate the flow of liquid material at start-up and the re-flow of the liquid metal at stopping and therefore keeps the reject rate high.